A chromate treatment has heretofore been known for improving close adhesion between a steel plate and an organic coating. Owing to its excellent corrosion resistance and close adhesion, therefore, the chromate treatment has been widely employed in the fields of domestic electric appliances, building materials, vehicles, aircraft, containers and the like.
If roughly classified from the standpoint of the treating method, the chromate treatments can be divided into those of the chemical conversion type (reaction type coating type) and those of the electrolysis type. If roughly classified from the standpoint of the formed films, the chromate treatments can be divided into those of the type in which hexavalent chromium remains in trace amounts in the final product to utilize the self-repairing effect to a large extent and those of the type in which no hexavalent chromium remains in the final product. Those of the type in which hexavalent chromium remains in trace amounts in the final product have now been blamed for their probability of permitting hexavalent chromium to elute out into the environment such as soil when they are disposed of. It is, therefore, a trend chiefly in European countries to abolish the use of the chromate treatment. Besides, in the chromate treatment of any type, hexavalent chromium is contained in the treatment liquid accompanied, therefore, by various problems from the standpoint of environment. Namely, it is essential to completely treat waste water and waste gases stemming from the hexavalent chromium-containing treating solution so that they are not drained to the exterior requiring, therefore, a huge sum of cost for the facilities for treating waste water and waste gases, and for the disposal is necessary. Besides, ever strict regulations have been imposed on transporting the sludge after having treated the waste water and on discharging the gases thereof. Therefore, it has been desired to develop a chromium-free surface treatment comparable to the conventional chromate treatment.
A metal material for metal containers has been treated with chromate in a manner that no hexavalent chromium finally remains, as a matter of course, and, besides, the metal material has, usually, been coated with an organic resin. For example, attempts have been made to cathodically electrolyze a tin-plated steel plate in an aqueous solution of sodium bichromate, to cathodically electrolyze the steel plate in an aqueous solution of a fluoride-containing anhydrous chromate, or to treat an aluminum alloy with chromic phosphate followed by coating with an organic resin.
To treat the surfaces of the steel plate without using chromium, a dip treatment has been proposed by using a treating solution containing Zr (zirconium) or Ti (titanium) (patent document 1).
However, the film on the surface-treated steel plate treated with Zr or Ti by dip treatment has poor corrosion resistance and deposits at a rate lower than that of the electrolytic chromate-treated steel plate (TFS) that has heretofore been used as a material for cans, and is, therefore, produced very inefficiently.
Therefore, treatments with Zr and/or Ti based on the cathodic electrolysis have been proposed as a high-speed treatment process to substitute for the dip treatment (patent documents 2 and 3).